Chapter 8
Understanding the Universe
We understood about motion and gravity. We discussed about the nature of light, which is a form of energy. We now know what heat is and what atoms actually are. All these things and all these quantities are a small part of the grand design of the universe. In this chapter, we get our attention toward the universe. First of all, we learn about the solar system.
Our solar system had 9 planets, but now, due to our definition of what a planet is, we have discarded Pluto as a planet and now call it a dwarf planet. Solar system is incomplete without sun, sun’s gravity holds this system together. All the planets, their moons, dwarf planets, comets, asteroids revolve around the sun.
Solar system is a system and it is moving as a system in the universe. Sun is not static, it is also revolving around the Milky Way galaxy’s centre. This system is a very small part of the galaxy. Aristotle had thought that sun, and all the other bodies moved around earth, but Galileo, Copernicus, Kepler, Newton and later scientists and observations firmly established that sun was the centre of the solar system. I am not going to discuss stupid facts like the diameter of the sun, or mass of the planets. These are not necessary, the truth is that sun is the most massive body in the solar system, thus, all bodies revolve around it. Jupiter is the second most massive body in the solar system. It is the largest planet in the solar system, and I hope you agree with me, that earth and Saturn are the two most beautiful planets in the solar system – earth because it sustains life and Saturn because of its beautiful rings.
Now we turn our attention on galaxies. Back in the 19th century, astronomers had discovered many nebulae in the universe. They thought that they were all parts of Milky Way. But Edwin Hubble measured distances to those so-called nebulae and they turned out to be distant galaxies! Hubble established the fact that the universe was full of galaxies. A galaxy is a group consisting of stars. Maybe Milky Way contains billions of stars. We do not know the exact number but we try to digest the fact that galaxies are so large that they must be home to at least a million stars, or maybe even more. Universe is very big. It is way beyond our imagination. It is home to millions of galaxies, we do know this. But what after that?
Motion is very much prevalent in the universe. We are moving, all the planets are moving and the sun is moving around the galaxy. It was assumed a century ago that the entire universe was static. It is hard to imagine an object to be at rest. It is always moving with respect to something. For example, a book kept on a table is at rest with respect to you, but it is in motion when Neil Armstrong sees it from the moon! All the planets are moving too, the sun is itself in motion. In fact, the entire universe is in motion – nothing is stationary. To know about this motion in detail, switch to the book Let us discover Physics: Appreciating Nature.
One thing that we know about the universe is that it is also in motion. It is expanding. To know about this expansion, you must know about Doppler Effect, which is explained very much in detail in the book ‘Let us discover Physics.’ But here we would like to know what causes this expansion? We ask ourselves: Will this expansion ever stop?
We think of two forces. One of the force is gravity, the force that we think could stop the expansion of the universe. The other force which we think could accelerate the expansion is unknown. This is opposing the action of gravity. If there is enough matter (enough mass) in the universe, gravity would ultimately cause the universe to stop expanding and shrink back to smaller sizes. On the other hand, the mystery force or dark force would cause the universe to expand forever. Both would ultimately lead to death of the universe.
How did the universe begin? Why do we study about the universe in a book about physics? What do we gain from it? Actually the universe is governed by four fundamental forces. These four are gravity, electromagnetism, weak and strong nuclear forces. To know more about these forces, you must read Let us discover Physics. But what are we trying to say? I am saying that if we keep on studying the universe, someday we may discover another fundamental force, which will change the way we look at the universe. Anything can happen and thus physics can change very rapidly. How did the universe begin? No one knows for sure. But one theory called the big bang theory possibly answers the questions of creation, not of creation, in fact about the beginning. The universe is currently expanding, we do not know what may happen at the next moment, but if we run the cosmic movie backward, we see that the galaxies get closer together. We ultimately get to a time when all the galaxies were exactly at the same location.
Maybe we are getting out of the scope of this book, but we do know one thing: Physics and universe get related in astrophysics. It is an adventurous journey of the entire human race.